This study is for clarifying the biochemical mode of action of selenium as an essential trace element, and as a modifier of environmental toxicants. Further chemical characterization of the forms and redox states of selenium in glutathione peroxidase will be undertaken. Further studies on the forms of selenium in marine food fish will be done, and biological availability of such compounds determined. Urinary metabolites of selenium will be isolated and characterized. Methodology for isolating selenium in forms suitable for characterization by mass spectrometry will be refined. The biosynthetic origin of urinary metabolites will be investigated. This work will provide a better understanding of the causes for low biological availability of selenium in marine fish, and permit studies to determine if selenium is the protective factor in tuna that decreases methylmercury toxicity. Better knowledge of forms of selenium that are consumed in foods and the metabolic fate of this selenium will provide a better basis for estimating the nutritional requirement for humans.